It means keep working your pattern stitch ( here, I believe it's stockinette) over the number of stitches in your row, without any shaping. Typically it's for a certain length, but it's also seen as a number of rows. Examples:

Work even in stockinette for 4".
If you have 20 stitches in your row, work stockinette over 20 stitches until you have 4" of stockinette from where you started.
Work even in pattern stitch for 20 rows.
If you have 20 stitches across your row in your pattern stitch, work 20 rows of it.